Oxygen delignification is a well-known process for removing lignin from wood pulp by treatment with oxygen and alkali followed by washing to remove soluble oxygen-lignin reaction products. The oxygen delignification reactions are typically carried out by mixing oxygen with medium consistency, heated alkaline pulp and passing the resulting mixture through a reactor with a sufficient contact time to allow the reaction to proceed to the desired degree. One type of reactor used for delignification is a vertical upflow reactor in which the pulp-oxygen mixture is introduced into the bottom of the reactor, flows upward while the reactions take place, and treated pulp is withdrawn from the top of the reactor.
The reactor feed mixture can be prepared by methods known in the art. U.S. Pat. No. 4,886,577 discloses the use of a specifically-designed centrifugal pump in which a pulp slurry is degassed by vacuum while passing through the pump, followed by addition of oxygen directly into the pulp at the pump discharge utilizing a shear plate or an oxygen permeable material which causes the oxygen to be introduced as small bubbles. South African Patent Application 868664 describes an alternate method to introduce oxygen into pulp which comprises passing the heated pulp in a completely fluidized state through an unobstructed flow path where it is contacted with highly dispersed oxygen bubbles ranging from 2 to 10 microns in diameter. This patent application also summarizes earlier alternative methods for oxygen dispersion described in the prior art. Australian Patent Application 22021/88 describes a similar method for introducing oxygen or oxygen-steam mixtures into the pulp.
In oxygen delignification at temperatures typically in the range of 80.degree. to 120.degree. C., the amount of oxygen required for delignification is much larger than the amount of oxygen soluble in the liquor associated with a given amount of pulp. In order to supply sufficient oxygen for delignification in the reactor, it is therefore necessary to incorporate bubbles of free oxygen gas in the pulp introduced to the reactor. It is desirable that these bubbles be very small in order to maximize the interfacial area so that additional oxygen can dissolve in the liquor as dissolved oxygen is consumed in the delignification reactions. The reactor system should be designed to achieve a constant upward flow velocity at all radial locations in the reactor, i.e., plug flow. Deviations from plug flow, in which some portions of the pulp move at a higher velocity and thus have less residence time in the reactor than other portions of the pulp, will cause uneven delignification and poor product quality. Careful design of the inlet and outlet sections of the reactor is necessary, since both sections influence pulp flow distribution throughout the reactor.
U.S. Pat. No. 5,034,095 discloses an upflow reactor for oxygen delignification comprising a cylindrical vessel having conical chambers connected to the inlet (bottom) and outlet (top) of the reactor wherein pulp is introduced and withdrawn at the axial center of the respective conical chambers. The convergence angle of each conical chamber, also defined as the included angle, is less than 60 degrees, preferably 20-60 degrees. No device to aid in pulp distribution or withdrawal is used in either the inlet chamber or the outlet chamber. This patent also describes a type of prior art reactor which utilizes a rotating mechanical distributor at the inlet and a mechanical discharge device at the outlet to aid in distribution and withdrawal of pulp from the reactor. These mechanical devices, which are widely used in commercial reactor systems, are effective for pulp feed distribution and withdrawal but can increase capital and maintenance costs for such reactor systems.
Improved reactor designs for oxygen delignification are desirable to achieve consistent product homogeneity and minimize the capital and operating costs of the reactor system. Such designs should emphasize operating simplicity and minimize complex design features. The reactor system of the present invention described and claimed below satisfies these requirements and offers improvements over prior art reactor systems.